Method for manufacturing bonded substrates and substrates for use in the bonded substrates

ABSTRACT

A method for manufacturing bonded substrates includes: forming the first terminals on the first substrate, the first terminals each having a metal core projecting from a surface of the first substrate, each metal core coated with a solder layer lower in a melting point than the metal core; forming the conductive second terminals on the second substrate; and electrically bonding the first terminals to the second terminals by heating the first and second substrates while applying pressure to the first substrate and the second substrate. In the forming of the first terminals, a ratio of a height of the metal core from the surface of the first substrate in a thickness direction of the first substrate to a thickness of the solder layer in the thickness direction of the first substrate is in a range of from 1:1 to 2:1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2005-99566 filed on Mar. 30, 2005, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for manufacturing bonded substrates inwhich first terminals formed on a first substrate are bonded to secondterminals formed on a second substrate, and a substrate for use in thebonded substrates.

2. Description of the Related Art

US 2004/113994 A discloses a method for manufacturing an inkjet head,including: transferring thermosetting epoxy-based resin (thermosettingadhesive) to a plurality of terminals (metal cores) having solder(solder layers) formed on their surfaces, and bonding the terminals toland portions (second terminals) formed on individual electrodes throughthe solder and the epoxy-based resin. For bonding the land portions tothe terminals in this method for manufacturing an inkjet head, first,the terminals to which the epoxy-based resin has been transferred arepressed onto the land portions. In this event, the epoxy-based resinmigrates to the peripheries of the land portions and the terminals, andis discharged from between the land portions and the terminals. Thus,the land portions, the terminals and the solder are surrounded by theepoxy-based resin. Next, the solder begins to melt due to heating, andthe terminals approach the land portions. Then, the epoxy-based resinsurrounding the land portions, the terminals and the solder is cured.When the solder is then cured, the terminals and the land portions arebonded so that they are electrically connected to each other. In thismanner, the epoxy-based resin is located to surround the land portions,the terminals and the solder. Thus, the molten solder can be restrainedfrom extending to the outside of the epoxy-based resin.

SUMMARY OF THE INVENTION

A result of researches made by the inventors shows, however, thataccording to the technique disclosed in US 2004/113994 A, some of alarge number of solders formed on the surfaces of the terminals (metalcores) may not be electrically bonded to the land portions, depending onkinds of the solders and bonding conditions.

The invention provides a method for manufacturing bonded substrates,capable of electrically bonding first terminals and second terminalswith high reliability, and a substrate for use in the bonded substrates.

According to one aspect of the invention, a method for manufacturingbonded substrates in which a plurality of first terminals formed on afirst substrate are electrically bonded to a plurality of secondterminals formed on a second substrate, includes: forming the firstterminals on the first substrate, each of the first terminals having ametal core projecting from a surface of the first substrate, each metalcore coated with a solder layer, the solder layer being lower in amelting point than the metal core; forming the conductive secondterminals on the second substrate; and electrically bonding the firstterminals to the second terminals by heating the first substrate and thesecond substrate while applying pressure to the first substrate and thesecond substrate. In the forming of the first terminals, a ratio of aheight of the metal core from the surface of the first substrate in athickness direction of the first substrate to a thickness of the solderlayer in the thickness direction of the first substrate is in a range offrom 1:1 to 2:1.

The present inventors carried out researches on the assumption thatsolder not electrically bonded with a land portion in the techniquedisclosed in US 2004/113994 A would be caused by some ratio of theheight of a metal core to the thickness of the solder. As a result, theinventors found that since the ratio of the core height (30 μm) indesign to the solder thickness (7-8 μm) in design in US 2004/113994 A isabout 3.75:1 to 4.29:1 is very large, the variation range of actual coreheight and the variation range of actual sum of the core height and thesolder layer thickness r overlap each other partially. That is, theinventors found that the maximum value of the actual core height islarger than the minimum value of the actual sum of the core height andthe solder layer thickness. Due to occurrence of such a phenomenon, aland portion and solder may be not electrically bonded with each otheraccording to the technique disclosed in US 2004/113994 A. In contrast,according to the above described method, the height of the metal core isset as a comparatively small value that is equal to or smaller than avalue twice as large as the thickness of the solder layer. Accordingly,the variation range of the actual metal core height hardly overlaps thevariation range of the actual sum of the metal core height and thesolder layer thickness. That is, the maximum value of the actual metalcore height is hardly larger than the minimum value of the actual sum ofthe metal core height and the solder layer thickness. It is thereforepossible to electrically bond the first terminals and the secondterminals with high reliability. In addition, there is no fear that theheight of the metal core is smaller than the thickness of the solderlayer. Accordingly, it is possible to prevent the adhesion between themetal core and the solder layer from lowering.

According to another aspect of the invention, a substrate includes asurface and a plurality of first terminals on the surface to beelectrically bonded to a plurality of second terminals formed on anothersubstrate. The first terminals are to be electrically bonded to aplurality of second terminals formed on another substrate. Each of thefirst terminals includes a metal core projecting from the surface. Eachmetal core is coated with a solder layer. The solder layer is lower inmelting point than the metal core. A ratio of a height of the metal corefrom the surface in a thickness direction of the substrate to athickness of the solder layer in the thickness direction of thesubstrate is in a range of from 1:1 to 2:1.

According to this configuration, the height of the metal core is set asa comparatively small value that is equal to or smaller than a valuetwice as large as the thickness of the solder layer. Accordingly, thevariation range of the actual metal core height hardly overlaps thevariation range of the actual sum of the metal core height and thesolder layer thickness. That is, the maximum value of the actual metalcore height is hardly larger than the minimum value of the actual sum ofthe metal core height and the solder layer thickness. It is thereforepossible to electrically bond the first terminals and the secondterminals with high reliability. In addition, there is no fear that theheight of the metal core is smaller than the thickness of the solderlayer. Accordingly, it is possible to prevent the adhesion between themetal core and the solder layer from lowering.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an outline perspective view of an inkjet head having bondedsubstrates manufactured by a manufacturing method according to a firstembodiment of the invention.

FIG. 2 is a sectional view taken along a line II-II in FIG. 1.

FIG. 3 is a plan view of a head body shown in FIG. 2, viewed from itstop.

FIG. 4 is an enlarged plan view of a region surrounded by a chain lineshown in FIG. 3.

FIG. 5 is a sectional view taken along a line V-V in FIG. 4.

FIG. 6A is a partially enlarged sectional view showing a bondingstructure between an actuator unit and an FPC, and FIG. 6B is apartially enlarged plan view of the actuator unit.

FIG. 7 is a graph showing variation ranges of a core height and aterminal height with respect to a ratio of the core height to a solderlayer thickness.

FIG. 8 is a graph showing a terminal destruction state after peelingtest for each predetermined ratio.

FIG. 9 is a flow chart of a manufacturing process of the inkjet head.

FIG. 10 is a flow chart of the manufacturing process of the inkjet head.

FIGS. 11A to 11D are application process views showing applying anadhesive to terminals of the FPC.

FIGS. 12A to 12C are bonding process views showing bonding between ahead body and the FPC.

FIG. 13 is a partially sectional view of an inkjet head manufactured bya manufacturing method for manufacturing bonded substrates according toa second embodiment of the invention.

FIG. 14 is a partially sectional view of an inkjet head manufactured bya method for manufacturing bonded substrates according to a modificationof the first embodiment of the invention.

FIG. 15A is a view showing a state where a core is formed on the FPC (ina through hole and on a lower surface of a cover film), and FIG. 15B isa view showing a state where a solder layer is formed on the core.

FIGS. 16A and 16B are schematic views of terminals 51 (51 a to 51 d).

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the invention will be described below with reference tothe drawings.

Description will be made on an inkjet head having bonded substratesmanufactured by a manufacturing method according to a first embodimentof the invention. Examples of a substrate according to this embodimentincludes a circuit board mounted with electronic components, a substratemounted with wires and terminals and a plate-like member havingterminals to be bonded to the terminals of those substrates. A flexibleprinted circuit (FPC) 50 functioning as a first substrate and anactuator unit 21 functioning as a second substrate (both of them will bedescribed later) are bonded to each other to form the bonded substrates.

FIG. 1 is a perspective view showing the external appearance of aninkjet head having the aforementioned bonded substrates. FIG. 2 is asectional view taken along a line II-II in FIG. 1. As shown in FIG. 1,an inkjet head 1 includes a head body 70 a base block 71 and the FPC 50.The head body 70 has a rectangular planar shape extending in a mainscanning direction. The base block 71 is disposed above the head body 70and has two ink reservoirs 3 formed therein. The FPC 50 is bonded to theupper surface of the head body 70.

The head body 70 includes a channel unit 4 in which ink channels areformed, and a plurality of actuator units 21 bonded to the upper surfaceof the channel unit 4 by a thermosetting adhesive, as shown in FIG. 2.The channel unit 4 has such a configuration that a plurality of thinsheets are laminated and bonded to one another. In addition, the bottomsurface of the head body 70 is formed as an ink ejection surface 70 a inwhich a large number of nozzles 8 (see FIG. 5) having very smalldiameters are formed. On the other hand, an FPC 50 having flexibility isbonded to the upper surface of each actuator unit 21, and withdrawn toleft or right, while being bent and withdrawn upward in FIG. 2.

FIG. 3 is a plan view of the head body 70 viewed from its top. As shownin FIG. 3, the channel unit 4 has a rectangular planar shape extendingin the main scanning direction. In FIG. 3, a manifold channel 5 providedin the channel unit 4 is depicted by broken lines. Ink is supplied fromthe ink reservoirs 3 of the base block 71 to the manifold channel 5through a plurality of openings 3 a. The manifold channel 5 branchesinto a plurality of sub-manifold channels 5 a extending in parallel tothe longitudinal direction (main scanning direction) of the channel unit4.

Four actuator units 21 each having a trapezoidal planar shape are bondedto the upper surface of the channel unit 4. The four actuator units 21are arrayed zigzag in two lines so as to avoid the openings 3 a. Eachactuator unit 21 is disposed so that its parallel opposite sides (upperand lower sides) extend in the longitudinal direction of the channelunit 4. The plurality of openings 3 a are arrayed in two lines in thelongitudinal direction of the channel unit 4 so that a total of tenopenings 3 a, that is, five in each line, are located not to interferewith the actuator units 21. Oblique sides of adjacent ones of theactuator units 21 overlap each other partially in the width direction(sub-scanning direction) of the channel unit 4.

In the ink ejection surface 70 a, an ink ejection region where the largenumber of nozzles 8 are arrayed in a matrix is formed so as tocorrespond to each region where the actuator unit 21 is bonded. Pressurechamber groups 9 are formed in the upper surface of the channel unit 4opposite to the actuator units 21. A large number of pressure chambers10 (see FIG. 5) are arrayed in a matrix in each pressure chamber group9. In other words, each actuator unit 21 has dimensions ranging over thelarge number of pressure chambers 10 constituting the correspondingpressure chamber group 9.

Returning to FIG. 2, the base block 71 is made of a metal material suchas stainless steel. Each ink reservoir 3 in the base block 71 is asubstantially rectangular parallelepiped hollow region extending in thelongitudinal direction of the base block 71. The ink reservoir 3 issupplied with ink from an ink tank (not shown) through an opening (notshown) provided at one end of the ink reservoir 3, so as to be alwaysfilled with the ink. The ink tank is installed externally. The inkreservoir 3 is provided with a total of ten openings 3 b arranged in twolines in the extending direction of the ink reservoir 3. The openings 3b are provided to make the ink flow out. The openings 3 b are disposedzigzag so as to be connected to the openings 3 a of the channel unit 4.That is, the ten openings 3 b of the ink reservoir 3 has the samepositional relationship as the ten openings 3 a of the channel unit 4 inplan view.

A lower surface 73 of the base block 71 projects downward in neighborportions 73 a of the openings 3 b with respect to their circumferences.The base block 71 abuts against neighbor portions of the openings 3 a inthe upper surface of the channel unit 4 only in the projecting neighborportions 73 a of the openings 3 b. Thus, any region other than theneighbor portions 73 a of the openings 3 b is separated from the headbody 70, and the actuator units 21 are disposed in these separatedregions.

A holder 72 includes a holding portion 72 a for holding the base block71, and a pair of projecting portions 72 b. The projecting portions 72 bare provided at a distance from each other in the sub-scanningdirection. The projecting portions 72 b project upward from the uppersurface of the holding portion 72 a. The base block 71 is fixedly bondedinto a recess portion formed in the lower surface of the holding portion72 a of the holder 72. Each FPC 50 bonded to the corresponding actuatorunit 21 is disposed to follow the surface of the correspondingprojecting portion 72 b of the holder 72 through an elastic member 83 ofsponge or the like. Driver ICs 80 are disposed on the FPCs 50 disposedon the surfaces of the projecting portions 72 b of the holder 72. Thatis, the FPCs 50 are electrically connected to a driver ICs 80 and theactuator units 21 of the head body 70 so that driving signals outputfrom the driver ICs 80 can be transmitted to the actuator units 21.

A substantially rectangular parallelepiped heat sink 82 is disposed inclose contact with the outside surface of each driver IC 80. As aresult, heat generated in the driver ICs 80 is dissipated by the heatsinks 82. A board 81 connected to the outside of each FPC 50 is disposedabove the corresponding driver IC 80 and the corresponding heat sink 82.Seal members 84 are put between the upper surface of the heat sink 82and the board 81 and between the lower surface of the heat sink 82 andthe FPC 50 respectively so as to prevent dust or ink from entering intothe body of the inkjet head 1.

FIG. 4 is an enlarged plan view of the region surrounded by a chain lineshown in FIG. 3. As shown in FIG. 4, in the regions of the channel unit4 opposite to the actuator units 21, four sub-manifold channels 5 aextend in parallel to the longitudinal direction (main scanningdirection) of the channel unit 4. A large number of individual inkchannels 7 (see FIG. 5) communicating with the nozzles 8 respectivelyare connected to each sub-manifold channel 5 a.

In the upper surface of the channel unit 4, a pressure chamber group 9including the large number of pressure chambers 10 each having a nearlyrhomboid planar shape is formed in a region opposite to each actuatorunit 21. The pressure chamber group 9 has a trapezoidal shapesubstantially as large as the outer shape of the actuator unit 21. Sucha pressure chamber group 9 is formed for each actuator unit 21. Eachpressure chamber 10 belonging to the pressure chamber group 9communicates with its corresponding nozzle 8 at one end of its longdiagonal, and communicates with the sub-manifold channel 5 a through anaperture 12 at the other end of the long diagonal. As will be describedlater, individual electrodes 35 (see FIGS. 6A and 6B) are arrayed in amatrix on the actuator unit 21 so as to be opposed to the pressurechambers 10, respectively. Each individual electrode 35 has a nearlyrhomboid shape in plan view and is one size smaller than the pressurechamber 10. Incidentally, in FIG. 4, the pressure chambers 10, theapertures 12 and the nozzles 8, which should be depicted by broken linesbecause they are located under the actuator units 21, are depicted bysolid lines in order to make the drawing understood easily.

Next, description will be made on the sectional structure of the headbody 70. FIG. 5 is a sectional view taken along a line V-V in FIG. 4,showing an individual ink channel. In this embodiment, each individualink channel 7 once runs upward, and reaches one end portion of acorresponding pressure chamber 10 formed in the upper surface of thechannel unit 4. Further, the individual ink channel 7 runs obliquelydownward from the other end portion of the pressure chamber 10 extendinghorizontally. Thus, the individual ink channel 7 is connected to acorresponding nozzle 8 formed in the lower surface of the channel unit4. As a whole, each individual ink channel 7 has an arched shape with acorresponding pressure chamber 10 disposed on its top. Thus, theindividual ink channels 7 can be disposed with high density, so that inkcan flow smoothly.

As is understood from FIG. 5, the head body 70 has a laminated structureconstituted by the actuator units 21 on the upper side and the channelunit 4 on the lower side. Both the units 4 and 21 are constituted bylaminating a plurality of thin sheets. Of these, in each actuator unit21, four piezoelectric sheets 41-44 (see FIGS. 6A and 6B) are laminated,and electrodes are disposed, as will be described in detail later. Ofthe piezoelectric sheets 41-44, only the uppermost layer is apiezoelectric layer having portions serving as active portions when anelectric field is applied thereto (hereinafter referred to as “layerhaving active portions”). The other three piezoelectric layers areinactive layers having no active portion.

On the other hand, the channel unit 4 is constituted by laminating atotal of nine sheet materials serving as a cavity plate 22, a base plate23, an aperture plate 24, a supply plate 25, manifold plates 26-28, acover plate 29 and a nozzle plate 30.

The cavity plate 22 is a metal plate in which a large number of nearlyrhomboid holes for forming spaces of the pressure chambers 10 areprovided within the range where the actuator unit 21 is pasted. The baseplate 23 is a metal plate in which for each pressure chamber 10 of thecavity plate 22, a communication hole between the pressure chamber 10and the aperture 12 and a communication hole between the pressurechamber 10 and the nozzle 8 are provided.

The aperture plate 24 is a metal plate in which for each pressurechamber 10 of the cavity plate 22 a communication hole between thepressure chamber 10 and the corresponding nozzle 8 is provided inaddition to a hole which will serve as the aperture 12. The supply plate25 is a metal plate in which for each pressure chamber 10 of the cavityplate 22 a communication hole between the aperture 12 and thesub-manifold channel 5 a and a communication hole between the pressurechamber 10 and the corresponding nozzle 8 are provided. Each of themanifold plates 26-28 is a metal plate in which for each pressurechamber 10 of the cavity plate 22 a communication hole between thepressure chamber 10 and the corresponding nozzle 8 is provided. Thecover plate 29 is a metal plate in which for each pressure chamber 10 ofthe cavity plate 22 a communication hole 29 a between the pressurechamber 10 and the corresponding nozzle 8 is provided. The nozzle plate30 is a metal plate in which a nozzle 8 is provided for each pressurechamber 10 of the cavity plate 22.

These nine plates 22-30 are aligned and laminated to one another so thatindividual ink channels 7 are formed as shown in FIG. 5. The nine platesconstituting the channel unit 4 are made of one and the same metalmaterial in this embodiment. Although SUS430 is used, another metalmaterial such as SUS316 or 42 alloy may be used. Alternatively, theplates 22-30 may be made of different metal materials.

As is apparent from FIG. 5, the pressure chambers 10 and the apertures12 are provided on different levels in the laminated direction of therespective plates. Consequently, in the channel unit 4 opposite to theactuator units 21, as shown in FIG. 4, an aperture 12 communicating withone pressure chamber 10 can be disposed in the same position as anotherpressure chamber 10 adjacent to the one pressure chamber 10 in planview. As a result, the pressure chambers 10 are brought into closecontact with one another and arrayed with higher density. Thus,high-resolution image printing can be attained by the inkjet head 1occupying a comparatively small area.

Next, description will be made about the bonding structure between theactuator unit 21 and the FPC 50. FIG. 6A is a partially enlargedsectional view showing the bonding structure between the actuator unit21 and the FPC 50. FIG. 6B is a partially enlarged plan view of theactuator unit 21.

As shown in FIG. 6A, the actuator unit 21 includes four piezoelectricsheets 41-44 each formed to have one and the same thickness of about 15μm. The piezoelectric sheets 41-44 are formed as continuous lamellarflat plates (continuous flat plate layers) to be disposed over a largenumber of pressure chambers 10 constituting a pressure chamber group 9.When the piezoelectric sheets 41-44 are disposed as continuous flatplate layers over a large number of pressure chambers 10, the individualelectrodes 35 can be disposed on the piezoelectric sheet 41 with highdensity, for example, by use of a screen printing technique.Accordingly, the pressure chambers 10 to be formed in positionscorresponding to the individual electrodes 35 can be also disposed withhigh density. Thus, high-resolution images can be printed. Thepiezoelectric sheets 41-44 are made of a lead zirconate titanate (PZT)based ceramics material having ferroelectricity.

In this embodiment, the individual electrodes 35 are formed only on thepiezoelectric sheet 41, which is the uppermost layer. A common electrode34 formed all over the sheet surface (all over the back surface of thepiezoelectric sheet 41) and having a thickness of about 2 μm is putbetween the piezoelectric sheet 41, which is the uppermost layer, andthe piezoelectric sheet 42, which is under the piezoelectric sheet 41.No electrode is disposed between the piezoelectric sheet 42 and thepiezoelectric sheet 43 or between the piezoelectric sheet 43 and thepiezoelectric sheet 44. The individual electrodes 35 and the commonelectrode 34 are made of a metal material such as Ag—Pd based metalmaterial.

As shown in FIG. 6B, each individual electrode 35 includes a mainelectrode region 35 a and an accessory electrode region 35 b. The mainelectrode region 35 a is disposed in a position where the main electroderegion 35 a is opposed to the pressure chamber 10. The accessoryelectrode region 35 b is connected to the main electrode region 35 a andled out from an acute portion of the main electrode region 35 a to aposition where the accessory electrode region 35 b does not face thepressure chamber 10. The main electrode region 35 a has a nearlyrhomboid planar shape, which is substantially similar to the pressurechamber 10, and whose salient portion is formed by a curved line. Asubstantially circular land 36 (functioning as a second terminal) isprovided in a position where the land 36 is in contact with the tip ofthe accessory electrode region 35 b. As shown in FIG. 6B, the land 36 isopposed to a region of the cavity plate 22 where no pressure chamber 10is formed. The land 36 is, for example, made of gold containing glassfrit. The land 36 is formed on the surface of an end portion of theaccessory electrode portion 35 b as shown in FIG. 6A. The commonelectrode 34 is grounded in a not-shown region. Consequently, the commonelectrode 34 is kept in constant potential or the ground potential inthis embodiment equally over the regions corresponding to all thepressure chambers 10.

As shown in FIG. 6A, the FPC 50 includes a base film 49, a plurality ofwires 48 made of copper foil and formed on the lower surface of the basefilm 49, and a cover film 40 covering substantially all the lowersurface of the base film 49. In the cover film 40, a through hole 45 isformed in a position facing one end portion of each wire 48. The endportion of each wire 48 has a circular shape with an enlarged diameter.The through hole 45 is formed to have a smaller diameter than thediameter of one end portion of each wire 48. That is, the outercircumferential edge portion of the one end portion of each wire 48 iscovered with the cover film 40 as shown in FIG. 6A. A terminal 51(functioning as a first terminal) of the FPC 50 is bonded to the one endportion of the wire 48 through the through hole 45. The other endportion of the wire 48 is electrically connected to the driver IC 80.

The base film 49 and the cover film 40 are sheet members havinginsulating properties. In the FPC 50 in this embodiment, the base film49 is made of polyimide resin, and the cover film 40 is made of aphotosensitive material. Both of the base film 49 and the cover film 40have flexibility. Since the cover film 40 is made of the photosensitivematerial, it is easy to form a large number of through holes 45.

The terminal 51 includes a core 52 (functioning as a metal core) havingelectric conductivity and a solder layer 53 having electricconductivity. For example, the core 52 includes or is made of nickel.The solder layer 53 is formed to cover the surface of the core 52. Theterminal 51 is formed to close the through hole 45 while covering theouter circumferential edge of the through hole 45 in a lower surface 40a of the cover film 40, so as to be convex toward the piezoelectricsheet 41. A thermosetting adhesive 54 is disposed in the outercircumference of the terminal 51 as shown in FIG. 6A. A tip portion ofthe solder layer 53 of the terminal 51 and the land 36 abut against eachother so as to be electrically connected with each other. Also, thethermosetting adhesive 54 surrounding the land 36 and the terminal 51bonds the piezoelectric sheet 41 with the lower surface 40 a of thecover film 40. The thermosetting adhesive 54 is an epoxy-based agenthaving insulating properties. Since the thermosetting adhesive 54surrounds the terminal 51 and the land 36 so as to bond them with eachother, the solder layer 53 melted by heating can be restrained fromflowing out to the periphery. In parallel therewith, the individualelectrodes 35 can be prevented from short-circuiting.

An Sn-3Ag-0.5Cu (tin-silver-copper) alloy is used for the solder layer53 in this embodiment. This alloy is a Pb free solder in which Cu(copper) is added to an Sn—Ag based (high-melting) solder and whosemelting point is 218° C. For example, the solder layer 53 may be formedout of an Sn—Ag based solder added with another metal such as Bi(bismuth). Further, not only three-element based alloys but alsofour-element based alloys may be used. An Sn—Cu based solder or an Sn—Sb(antimony) based solder other than the Sn—Ag based solder may be used asthe high melting solder. The solder layer 53 may be formed out of not ahigh-melting solder but a middle-melting solder or a low-melting solder.An Sn—Zn (zinc) based solder may be used as the middle-melting solder.An Sn—Bi based solder or an Sn—In (indium) based solder may be used asthe low-melting solder. These middle-melting solder and the low-meltingsolder may be three-element based or four-element based solders addedwith other metals. When the core 52 is made of nickel, the adhesionbetween the core 52 and the solder layer 53 made of such a material canbe improved.

The FPC 50 has a large number of terminals 51. The terminals 51 aredesigned to correspond to the lands 36 one by one. Accordingly, theindividual electrodes 35 electrically connected to the lands 36respectively are connected to the driver ICs 80 through the wires 48independent of one another in the FPC 50, respectively. Thus, thepotential of each individual electrode 35 can be controlled individuallyfor its corresponding pressure chamber 10.

Here, description will be made below on a relationship between a ratioof a height of the core 52 from the lower surface 40 a to a thickness ofthe solder layer 53 and variations of a core height and a terminalheight, which are generated when the core 52 and the solder layer 53 areformed. FIG. 7 is a graph showing the variation range of the height ofthe core 52 and the variation range of the height of the terminal 51(variation range of a sum of the height of the core 52 and the thicknessof the solder layer 53) with respect to each ratio of the height of thecore 52 to the thickness of the solder layer 53 when the projectingheight of the terminal 51 from the lower surface 40 a is about 40 μm.The ordinate in FIG. 7 designates the height, and the abscissadesignates the ratio of the core height to the solder layer thickness.The variation range of the core height and the variation range of theterminal height are expressed by values obtained by measuring 20 coresand 20 terminals produced for each ratio of the core height to thesolder layer thickness.

Here, a method for measuring the height of the core 52 from the lowersurface 40 a of the cover film 40 (a surface of the FPC 50), thethickness of the solder layer 53 in the thickness direction of the FPC50 and the height of the terminal 51 in the thickness direction of theFPC 50 will be described. As detailed later, the core 52 is formed inthe through hole 45 and on the lower face 40 a of the cover film 40 bythe electroplating method (see FIG. 15A), and then, the solder layer 53is formed on the core 52 by the electroplating method (see FIG. 15B). Asshown in FIG. 15A, a contour of the grown core 52 has three portions,that is, a straight portion S and curve portions R1, R2 on both sides ofthe straight portion S. The straight portion S connects the curveportions R1, R2. The electroplating method grows the core 52 (e.g.,nickel) isotropically. Therefore, the inventors assumed that:

-   -   (i) the curve portions R1, R2 are circular arcs having the same        curvature radius r,    -   (ii) the straight portion S is in parallel to the lower surface        40 a of the cover film 40, and    -   (iii) a width of the straight portion S is equal to a width w of        the through hole 45.        As apparent from FIG. 15A, the height h of the core 52 is equal        to the curvature radius r of the curve portions R1, R2. Since        the through hole 45 is formed by the photolithography method (as        described later), a width w of the through hole 45 is known in        advance. After the core 52 is formed in the through hole 45 and        on the lower surface 40 a of the cover film 40, the width x of        the core 52 from one end on the curve-portion R1 side to the        other end on the curve-portion R2 side is measured using an        optical microscope NEXIV VM-500N produced by Nikon Corporation.        Specifically, the core 52 is observed two-dimensionally from        above using the optical microscope to measure the width x of the        core 52. Alternatively, this measurement may be done using a        scanning electron microscope. Then, the height h of the core 52        can be obtained by the expression (1).

$\begin{matrix}{h = \frac{x - w}{2}} & (1)\end{matrix}$Alternatively, a contact-type step meter may be used to measure theheight h of the core 52.

Thereafter, the solder layer 53 is formed on the core 52 height of whichhas already been known, by the electroplating method. A contour of thesolder layer 53 (i.e., a contour of the terminal 51) also has threeportions, that is, a straight portion S′ and curve portions R1′, R2′ onboth sides of the straight portion S′. The straight portion S′ connectsthe curve portions R1′, R2′. The electroplating method also grows thesolder layer 53 (e.g., SnAgCu alloy) isotropically. Therefore, theinventors assumed that:

-   -   (iv) the curve portions R1′, R2′ are circular arcs having the        same curvature radius r′,    -   (v) the straight portion S′ is in parallel to the lower surface        40 a of the cover film 40 and the straight portion S of the core        52, and    -   (vi) a width of the straight portion S′ is equal to the width w        of the through hole 45.        As apparent from FIG. 15B, the height (t+h) of the terminal 51        from the lower surface 40 a of the cover film 40 is equal to the        curvature radius r′ of the curve portions R1′, R2′. After the        solder layer 53 is formed on the core 52, a width y of the        terminal 51 from one end on the curve-portion R1′ side to the        other end on the curve-portion R2′ side is measured using the        optical microscope NEXIV VM-500N. Specifically, the terminal 51        (solder layer 53) is observed two-dimensionally from above using        the optical microscope to measure the width y of the terminal        51. Alternatively, this measurement may be done using the        scanning electron microscope. Then, the height (t+h) of the        terminal 51 is obtained by the expression (2).

$\begin{matrix}{{t + h} = \frac{y - w}{2}} & (2)\end{matrix}$Alternatively, the contact-type step meter may be used to measure theheight (t+h) of the terminal 51.

Finally, since the height h of the core 52 and the height (t+h) of theterminal 51 have already been known, the thickness t of the solder layer53 in the thickness direction of the FPC 50 can be obtained by theexpression (3).

$\begin{matrix}{t = {\frac{y - w}{2} - h}} & (3)\end{matrix}$

Returning to FIG. 7, the lower limit value (the lower limit value of thevariation) of the height of the terminal 51 and the upper limit value(the upper limit value of the variation) of the height of the core 52begin to overlap each other at the point where the ratio of the coreheight to the solder layer thickness is 1.95:1, that is, the ratio isabout 2:1. When the ratio is higher than 2:1, there is a tendency forthe variation range of the height of the terminal 51 and the variationrange of the height of the core 52 to overlap each other largely. On thecontrary, when the ratio is equal to or less than 2:1, the variationrange of the height of the terminal 51 and the variation range of theheight of the core 52 hardly overlap each other.

The relation between the height of the terminal 51 and the height of thecore 52 will be described with reference to FIG. 16. FIGS. 16A and 16Bare schematic views of terminals 51. For the convenience of thedescription, the thermosetting adhesive 54 is omitted in FIGS. 16A and16B. In FIG. 16A, a terminal 51 a has the minimum height (t+h)_(min)among terminals 51 formed on the same substrate (e.g., the FPC 50), anda core 52 of a terminal 51 b has the maximum height h_(max) among thecores 52 of the terminals 51 formed on the same substrate. The terminals51 a and 51 b satisfy a relation that:h _(max)≦(t+h)_(min)  (4)In other words, FIG. 16A shows the state where the variation range ofthe height of the terminal 51 and the variation range of the height ofthe core 52 does not overlap each other.

When the actuator unit 21 (the lands 36; not shown in FIG. 16A) and theFPC 50 (the terminals 51) are bonded to each other, the terminals 51 aand 51 b are pressed toward and against the actuator unit 21.Specifically, at first, the top of the solder layer 53 of the terminal51 b abuts against a corresponding land 36 of the actuator unit 21 andthe solder layer 53 of the terminal 51 b begins to deform due toreactive force from the land 36 of the actuator unit 21 while theterminal 51 a is in contact with no land 36. Since the solder layer 53is relatively soft, the reactive force deforms the solder layer 53 b ofthe terminal 51 b. The core 52 of the terminal 51 b approaches theactuator unit 21 while the solder layer 53 of the terminal 51 b isdeforming. When the solder layer 53 of the terminal 51 a abuts againstthe actuator unit 21, the solder layer 53 of the terminal 51 a alsobegins to deform due to the reaction force from the actuator unit 21.Finally, the actuator unit 21 is located at a certain level L₂. In otherwords, as a result of the bonding processing of the actuator unit 21 andthe FPC 50, the highest level (L₁) of the terminals is come down to thecertain level L₂. FIG. 16A shows the case where the level L₂ isidentical with the top of the core 52 of the terminal 51 b. Since abroken line indicating the level L₂ intersects both the terminals 51 aand 51 b, the actuator unit 21 can electrically contact with both theterminals 51 a and 51 b surely. That is, when the variation range of theheight of the terminal 51 and the variation range of the height of thecore 52 does not overlap each other, the lands 36 of the actuator unit21 can electrically contact with the terminals 51 of the FPC 50reliably.

Returning to FIG. 7, it is therefore understood that it is preferablethat the upper limit of the ratio of the height of the core 52 to thethickness of the solder layer 53 is equal to or lower than 2:1. Theratio may be equal to or less than 1.8:1. When the ratio is 2:1, thevariation ranges of the both overlap each other just slightly asdescribed above. Therefore, if the terminals 51 are formed on asubstrate, which is too hard to bend, the terminals 51 and the lands 36may be prevented from being electrically bonded with each other.However, since the terminals 51 are formed on the FPC 50 havingflexibility in this embodiment, the FPC 50 bends lightly so that theterminals 51 approach the lands 36 when the terminals 51 and the lands36 are bonded with each other. Thus, even if the ratio is 2:1, theterminals 51 and the lands 36 can be electrically bonded with each othersurely also due to the effect of absorbing the variation of the solderlayer 53 as will be described later.

On the other hand, in FIG. 16B, a terminal 51 c has the minimum height(t+h)_(min) among terminals 51 formed on the same substrate (e.g., theFPC 50), and a core 52 of a terminal 51 d has the maximum height h_(max)among the cores 52 of the terminals 51 formed on the same substrate. Theterminals 51 c and 51 d satisfy a relation that:h _(max)>(t+h)_(min)  (5)In other words, FIG. 16B shows the state where the variation range ofthe height of the terminal 51 and the variation range of the height ofthe core 52 overlaps each other.

In this case, even if the substrate (the terminals 51 c, 51 d) ispressed until the core 52 of the terminal 51 d abuts against theactuator unit 21, the actuator unit 21 (the land 36) does not contactwith the terminal 51 c located at the level L₃′, which is sufficientlyseparate from the level L₂′. It is noted that since the cores 52 of theterminals 51 are relative hard, the cores 52 of the terminals 51 don'tdeform only by pressing the terminals 51 against the actuator unit 21.Accordingly, when the variation range of the height of the terminal 51and the variation range of the height of the core 52 overlap each other,the lands 36 of the actuator unit 21 may not electrically contact withthe terminals 51 of the FPC 50.

Returning to FIG. 7, when the ratio is much higher than 2:1, thevariation ranges of the both overlap each other largely as describedabove. Even if the FPC 50 bends slightly, the overlapping of thevariation ranges cannot be absorbed by the effect of absorbing thevariation of the solder layer 53 and the bending of the FPC 50. Thus,some terminals 51 and some lands 36 may be not bonded in contact witheach other, or may be bonded insufficiently.

When the ratio is equal to or lower than 1.8:1, the variation ranges ofthe both do not overlap each other. Accordingly, the terminals 51 andthe lands 36 can be bonded in contact with each other surely also due tothe effect of absorbing the variation of the solder layer 53 even if thebending effect of the FPC 50 cannot be expected so much.

FIG. 8 shows a graph showing the terminal destruction state after apeeling test for each predetermined ratio. In FIG. 8, the ordinatedesignates the rates occupied by first to third modes, and the abscissadesignates the ratio of the core height to the solder layer thickness.

The peeling test herein is a destructive test as follows. That is, afterthe terminal 51 and the land 36 are bonded with each other, a sharpknife is used to scratch the periphery of the boundary between theterminal 51 and the FPC 50 to peel the FPC 50 from the actuator unit 21.The terminal condition of the FPC 50 is observed visually, and it isdetermined which mode the terminal condition has fallen into, a firstmode, a second mode or a third mode. The first mode designates the casewhere one end portion of the wire 48 bonded with the terminal 51 ispeeled from the FPC 50 when the terminal 51 bonded with the land 36 ispeeled off. The second mode designates the case where the terminal 51 ispeeled near the boundary between the terminal 51 and the wire 48 whenthe terminal 51 bonded with the land 36 is peeled off. The third modedesignates the case where the terminal 51 is peeled near the boundarybetween the core 52 and the solder layer 53 when the terminal 51 bondedwith the land 36 is peeled off.

Description will be made on the quality in each mode. When the peelingcondition falls into the first mode, it is judged that bonding betweenthe core 52 and the solder layer 53, that between the terminal 51 andthe land 36 and that between the terminal 51 and the wire 48 are firm.Thus, the terminal formation state is good. When the peeling conditionfalls into the second mode, it is judged that bonding strength betweenthe terminal 51 and the wire 48 is smaller than that sin the first mode,but that bonding between the core 52 and the solder layer 53 and thatbetween the terminal 51 and the land 36 are firm. Thus, the terminalformation state is practicable and good. However, when the peelingcondition falls into the third mode, it is judged that adhesion forcebetween the core 52 and the solder layer 53 is small due to the smallsurface area of the core 52 bonded with the solder layer 53. Thus, theterminal formation state is bad.

FIG. 8 shows the shares of the first to third modes (hatched areas shownin FIG. 8) obtained as follows. For each predetermined ratio of theheight of the core 52 to the thickness of the solder layer 53, thepeeling test was performed. It was then determined which mode the statesof a plurality of terminals belong to, the first mode, the second modeor the third mode. After that, the number of terminals regarded asdestroyed in the first mode is divided by the total number of terminals.Values obtained thus for the ratios are plotted by black points. Thenumber of terminals regarded as destroyed in the first and second modesis divided by the total number of terminals. Values obtained thus forthe ratios are plotted by triangular points. The black points areconnected to one another by a line, and the triangular points areconnected to one another by a line, so as to show the shares of thefirst to third modes (the hatched regions shown in FIG. 8).

As shown in FIG. 8, when the ratio of the core height to the solderlayer thickness is 5.85:1, destruction in the first mode occupies 53%when the terminals 51 bonded to the lands 36 are peeled. Destruction inthe second mode occupies the rest 47%. When the ratio of the core heightto the solder layer thickness is 3.56:1, destruction in the first modeoccupies 68%, and destruction in the second mode occupies the rest 32%.When the ratio of the core height to the solder layer thickness is1.5:1, destruction in the first mode occupies 55%, and destruction inthe second mode occupies the rest 45%. When the ratio of the core heightto the solder layer thickness is 0.59:1, destruction in the first modeoccupies 5%, and destruction in the third mode occupies the rest 95%.When the ratio of the core height to the solder layer thickness is0.36:1, destruction in the third mode occupies 100%. Thus, destructionin the first mode decreases suddenly as the ratio goes from 1.5:1 to1:1. In the point where the lines connecting the black points and thetriangular points cross each other, that is, when the ratio of the coreheight to the solder layer thickness is 1:1, destruction in the firstmode becomes 30%, and destruction in the second mode becomes 70%. Asshown in FIG. 8, when the ratio is lower than 1:1, all destructionexcept that in the first mode occurs in the third mode. On the contrary,when the ratio is not lower than 1:1, all destruction except that in thefirst mode occurs in the second mode.

It is therefore understood from FIG. 8 that the lower limit value of theratio of the core height to the solder layer thickness is 1:1. When theratio of the core height to the solder layer thickness is equal to orlarger than 1:1, destruction occurs only in the first mode and thesecond mode. Accordingly, each terminal 51 reaches a practicable level.On the contrary, when the ratio is lower than 1:1, destruction in thefirst mode decreases while destruction in the third mode increases.Thus, some terminals 51 do not reach the practicable level. Therefore,when the lower limit value of the ratio of the core height to the solderlayer thickness is set at 1:1, the state where the terminals 51 areformed is improved. That is, when the ratio of the height of the core 52to the thickness of the solder layer 53 in each terminal 51 is equal toor larger than 1:1, the surface area of the core 52 closely adhering tothe solder layer 53 can be kept large enough to keep the adhesionbetween the core 52 and the solder layer 53 higher than the bondingstrength between the core 52 and the wire 48. If this ratio were lowerthan 1:1, the surface area of the core 52 could not be kept large enoughto keep the adhesion the core 52 and the solder layer 53 higher than thebonding strength between the core 52 and the wire 48. That is,destruction would occur in the third mode.

In the plurality of terminals 51 of this embodiment, the core 52 isaveragely about 24.8 μm high from the lower surface 40 a, and the solderlayer 53 is averagely 16.5 μm thick. That is, the ratio of the height ofthe core 52 to the thickness of the solder layer 53 in each terminal 51is about 1.5:1. When the ratio between the core 52 and the solder layer53 constituting the terminal 51 is 1.5:1, the ratio is put within arange between the upper limit value 1.8:1 and the lower limit value 1:1,and destruction in the first mode reaches 50% or higher. Accordingly,when the terminals 51 are formed, there is no fear that the variationranges of the cores 52 and the solder layers 53 overlap each other.Further, the terminals 51 have good bonding conditions between theterminals 51 and the lands 36. Thus, the reliability of the terminals 51is improved. In addition, it will go well if the ratio of the coreheight to the solder layer thickness is in a range of from 1:1 to 2:1.Accordingly, when the terminal height is set at about 40 μm, thethickness of the solder layers 53 may be set in a range of from about13.3 μm to about 20 μm. When the thickness of the solder layers 53 isset within this range, it is possible to improve the effect of absorbingvariations caused by the softening of the solder layers 53 when theterminals 51 and the lands 36 are bonded with each other. That is, thesolder layers 53 become thicker than in the aforementioned publication,so that the range where the variation of the terminal height can beabsorbed can be expanded.

Next, description will be made on a method for driving each actuatorunit 21. The piezoelectric sheet 41 in the actuator unit 21 has apolarizing direction in the thickness direction of the piezoelectricsheet 41, which is the same as the direction to apply an electric field.That is, the actuator unit 21 has a so-called unimorph typeconfiguration in which one piezoelectric sheet 41 on the upper side(that is, distant from the pressure chambers 10) is set as a layer whereactive portions exist, while three piezoelectric sheets 42-44 on thelower side (that is, close to the pressure chambers 10) are set asinactive layers. Accordingly, when the individual electrodes 35 are setat positive or negative predetermined potential, eachelectric-field-applied portion between electrodes in the piezoelectricsheet 41 will act as an active portion (pressure generating portion) soas to contract in a direction perpendicular to the polarizing directiondue to piezoelectric transversal effect.

On the other hand, since the piezoelectric sheets 42-44 are not affectedby any electric field, they are not displaced voluntarily. Therefore,between the upper piezoelectric sheet 41 and each lower piezoelectricsheet 42-44, there occurs a difference in strain in a directionperpendicular to the polarizing direction, so that the piezoelectricsheets 41-44 as a whole try to be deformed to be convex on the inactiveside (unimorph deformation). In this event, as shown in FIG. 6A, thelower surface of the actuator unit 21 constituted by the piezoelectricsheets 41-44 is fixed to the upper surface of a diaphragm (cavity plate22) which defines the pressure chambers. Consequently, the piezoelectricsheets 41-44 are deformed to be convex on the pressure chamber side.Accordingly, the volume of each pressure chamber 10 is reduced so thatthe pressure of ink increases. Thus, the ink is discharged from thecorresponding nozzle 8. After that, when the individual electrodes 35are restored to the same potential as the common electrode 34, thepiezoelectric sheets 41-44 are restored to their initial shapes so thatthe volume of each pressure chamber 10 is restored to its initialvolume. Thus, the pressure chamber 10 sucks ink from the sub-manifoldchannel 5.

According to another driving method, each individual electrode 35 may beset at potential different from the potential of the common electrode 34in advance. In this method, the individual electrode 35 is once set atthe same potential as the common electrode 34 whenever there is adischarge request. After that, the individual electrode 35 is set atpotential different from the potential of the common electrode 34 againat predetermined timing. In this case, the piezoelectric sheets 41-44are restored to their initial shapes at the timing when the individualelectrode 35 has the same potential as that of the common electrode 34.Thus, the volume of the pressure chamber 10 increases in comparison withits initial volume (in the state where the individual electrode 35 andthe common electrode 34 are different in potential), so that ink issucked into the pressure chamber 10 through the sub-manifold channel 5.After that, the piezoelectric sheets 41-44 are deformed to be convex onthe pressure chamber 10 side at the timing when the individual electrode35 is set at different potential from that of the common electrode 34again. Due to reduction in volume of the pressure chamber 10, thepressure on ink increases so that the ink is discharged. In such amanner, ink is discharged from the nozzles 8 while the inkjet head 1 ismoved suitably in the main scanning direction. Thus, a desired image isprinted on paper.

Next, a method for manufacturing the aforementioned inkjet head 1 willbe described below. FIGS. 9 and 10 are flow charts of a manufacturingprocess of the inkjet head 1. To manufacture the inkjet head 1, partssuch as the channel unit 4 and the actuator units 21 are producedindividually, and the respective parts are assembled. As shown in FIG.9, first, the channel unit 4 is produced in Step 1 (S1). To produce thechannel unit 4, etching is performed upon the respective plates 22-30constituting the channel unit 4, with patterned photo-resist as mask.Thus, holes as shown in FIG. 5 are formed in the respective plates22-30. After that, the nine plates 22-30 aligned to form the individualink channels 7 are laminated on one another through a thermosettingadhesive. The nine plates 22-30 are heated to a temperature equal to orhigher than the curing temperature of the thermosetting adhesive whilepressure is applied thereto. As a result, the thermosetting adhesive iscured so that the nine plates 22-30 are fixedly bonded with one another.Thus, the channel unit 4 as shown in FIG. 5 can be obtained.

On the other hand, to produce each actuator unit 21, first, a pluralityof green sheets of piezoelectric ceramics are prepared in Step 2 (S2).The green sheets are formed in advance in expectation of shrinkagecaused by baking. Conductive paste is screen-printed on a part of thegreen sheets in accordance with the pattern of the common electrode 34.The green sheets are aligned with one another by use of a jig, so thatthe green sheet in which the conductive paste has been printed inaccordance with the pattern of the common electrode 34 is laid under oneof the green sheets in which the conductive paste has not been printed,and the other two of the green sheets in which the conductive paste hasnot been printed are laid further thereunder.

In Step 3 (S3), the laminate obtained in Step 2 is degreased in the samemanner as known ceramics, and further baked at a predeterminedtemperature. Consequently, the four green sheets are formed as thepiezoelectric sheets 41-44, and the conductive paste is formed as thecommon electrode 34. After that, conductive paste is screen-printed onthe uppermost piezoelectric sheet 41 in accordance with the pattern ofthe individual electrodes 35. The laminate is heated to bake theconductive paste. Thus, the individual electrodes 35 are formed on thepiezoelectric sheet 41. After that, gold containing glass frit isprinted on the tip portions of the accessory electrode regions 35 b ofthe individual electrodes 35 so as to form the lands 36 (forming of asecond terminal). In such a manner, the actuator units 21 as shown inFIG. 6A can be produced.

The channel unit production step of Step 1 and the actuator unitproduction step of Steps 2-3 are performed independently. Any one of thesteps may be performed previously, or the both may be performed inparallel.

Next, in Step 4 (S4), by use of a bar coater, a thermosetting adhesivewhose thermocuring temperature is about 80° C. is applied onto the uppersurface of the channel unit 4 obtained in Step 1, where a large numberof openings of the pressure chambers 10 have been formed. For example, atwo-component mixing type agent is used as the thermosetting adhesive.

Subsequently in Step 5 (S5), the actuator units 21 are mounted on thethermosetting adhesive applied to the channel unit 4. In this event,each actuator unit 21 is supported on beam portions among the pressurechambers 10 and positioned with respect to the channel unit 4 so thatthe individual electrodes 35 and the pressure chambers 10 are opposed toeach other. This positioning is performed based on positioning marks(not shown) formed in the channel unit 4 and the actuator units 21 inthe production steps (Step 1 to Step 3) in advance.

Next, in Step 6 (S6), the laminate of the channel unit 4, thethermosetting adhesive between the channel unit 4 and the actuator units21, and the actuator units 21 is pressurized and heated to a temperatureequal to or higher than the curing temperature of the thermosettingadhesive by a not-shown heating/pressurizing apparatus. Consequently,the openings of the pressure chambers 10 are closed by the actuatorunits 21. In Step 7 (S7), the laminate taken out from theheating/pressurizing apparatus is cooled naturally. Thus, the head body70 constituted by the channel unit 4 and the actuator units 21 aremanufactured.

Subsequently, to produce the FPC 50, first in Step 8 (S8), the base film49 made of polyimide resin is prepared, and copper foil is pasted allover one surface of the base film 49 through an adhesive, as shown inFIG. 10. In Step 9 (S9), photo-resist is patterned and formed on thesurface of the copper foil. Etching is performed upon the copper foilwith the photo-resist as mask. Thus, the copper foil other than portionsserving as the plurality of wires 48 is removed. After that, thephoto-resist is removed. Next, in Step 10 (S10), the photosensitivecover film 40 is pasted through an adhesive to the surface of the basefilm 49 where the wires 48 have been formed. In Step 11 (S11), thethrough holes 45 are formed in positions of the cover film 40 opposed tothe lands 36 and opposed to one-end portions of the wires 48,respectively. In this event, a mask having a pattern corresponding tothe through holes 45 is used for forming the through holes 45 by thephotolithography method similar to that for forming the wires 48. Next,in Step 12 (S12), the cores 52 made of nickel are formed in the one-endportions of the wires 48 exposed from the through holes 45 by theelectroplating method so that the cores 52 project over the lowersurface 40 a of the cover film 40. The solder layers 53 made of SnAgCualloy are formed on the surfaces of the cores 52 by the electroplatingmethod. Thus, the terminals 51 electrically connected to the wires 48respectively are formed (forming a first terminal). In this event, theterminals 51 are formed so that the ratio between the core 52 and thesolder layer 53 forming each terminal 51 is in a range of from 1:1 to2:1, preferably in a range of from 1:1 to 1.8:1 as described above. Inthis embodiment, each terminal 51 is formed by the electroplating methodso that the tip of the core 52 is about 25 μm high from the lowersurface 40 a, and the solder layer 53 is about 15 μm thick. Due tovariations occurring in the height and the thickness, the core 52 isaveragely about 24.8 μm high, and the solder layer 53 is averagely about16.5 μm thick as described above. The ratio of the core height to thesolder layer thickness in each terminal 51 in the FPC 50 is in a rangeof from 1:1 to 2:1 and also is in a range of from 1:1 to 1.8:1. Thus,the FPC 50 is manufactured.

Next, description will be made on a method for applying thethermosetting adhesive 54 to the terminals 51 of the FPC 50. FIGS. 11Ato 11D are application process views in which the adhesive is applied tothe terminals 51 of the FPC 50. Prior to application of thethermosetting adhesive 54 to the terminals 51 of the FPC 50, first, inStep 13 (S13), a thermosetting adhesive layer 60 having a uniformthickness is formed on a resin sheet (plate material) 56 (forming anadhesive layer). As shown in FIG. 11A, a large amount of a thermosettingadhesive is disposed on a flat upper surface 57 of the resin sheet 56mounted on a stage 55. A squeegee 58 is then moved from left to right inFIG. 1A. Consequently, as shown in FIG. 11B, the thermosetting adhesivelayer 60 having a predetermined thickness is formed on the resin sheet56. In this event, the thermosetting adhesive layer 60 is formed so thatits thickness is slightly smaller than the projecting height of theterminals 51 from the lower surface 40 a of the cover film 40. In Step14 (S14), as shown in FIG. 11C, the FPC 50 is sucked on a suctionportion 91, and then disposed so that the terminals 51 face thethermosetting adhesive layer 60.

Next, in Step 15 (S15), the suction portion 91 is moved to approach thestage 55 till the tips of the terminals 51 abut against the resin sheet56. Thus, the terminals 51 are soaked in the thermosetting adhesivelayer 60. As shown in FIG. 1D, the suction portion 91 is moved to leavethe stage 55 so that the terminals 51 are separated from thethermosetting adhesive layer 60. Thus, the thermosetting adhesive 54 isapplied to the terminals 51 (adhesive application step). In this event,the thickness of the thermosetting adhesive layer 60 is slightly smallerthan the height of the terminals 51. Therefore, a gap is formed betweenthe lower surface 40 a of the cover film 40 (FPC 50) and thethermosetting adhesive layer 60 though the terminals 51 have been soakedin the thermosetting adhesive layer 60. As a result, the thermosettingadhesive of the thermosetting adhesive layer 60 is prevented fromadhering to the lower surface of the FPC 50, that is, the lower surface40 a of the cover film 40.

The aforementioned process from Step 8 to Step 15 may be performed priorto or in parallel with the forming of the channel unit 4 and theactuator units 21.

Subsequently, description will be made on a process for bonding the headbody 70 and the FPC 50. FIGS. 12A to 12C are process views in which thehead body 70 and the FPC 50 are bonded with each other. To bond eachactuator unit 21 of the head body 70 with the FPC 50, in Step 16 (S16),as shown in FIG. 12A, the FPC 50 retained by the suction portion 91 ispositioned so that the lands 36 face the terminals 51.

Next, in Step 17 (S17), as shown in FIG. 12B, the suction portion 91 ismoved downward so as to press each terminal 51 onto its correspondingland 36. At the same time, the FPC 50 and the head body 70 are heated bya not-shown heating apparatus so as to temporarily bond the terminals 51with the lands 36 (bonding step). At this pressing time, thethermosetting adhesive 54 applied to the surface of the terminal 51moves to surround the terminal 51 and the land 36 as shown in FIG. 12B.At the heating time, the FPC 50 and the head body 70 are heated to atemperature equal to or higher than the curing temperature of thethermosetting adhesive 54 and lower than the melting points of thesolder layer 53 and the core 52 of the terminal 51, for example, to 200°C. The temperature is kept constant for a predetermined time. As aresult, the solder layer 53 of the terminal 51 is softened so that thesolder layer 53 is deformed from the state shown in FIG. 12B to thestate shown in FIG. 12C. Accordingly, the contact area between theterminal 51 and the land 36 increases. Further, in this state, thethermosetting adhesive 54 surrounding the terminal 51, the land 36 andthe solder layer 53 is cured. In this event, the FPC 50 has flexibility.Even when the ratio of the height of the core 52 to the thickness of thesolder layer 53 is the upper limit value 2:1, all the terminals 51approach the lands 36 and abut against the lands 36 due to the softeningof the solder layer 53 and the bending of the FPC 50. Thus, when the FPC50 and the actuator unit 21 are cooled naturally in Step 18 (S18) wherea predetermined time has passed, the terminals 51 and the lands 36 areelectrically connected with each other, and the actuator unit 21 and theFPC 50 are bonded with each other by the thermosetting adhesive 54.

After that, the aforementioned inkjet head 1 is finished after thebonding step of the base block 71 and so on. Optionally if appropriate,the FPC 50 may be bonded in the aforementioned manner, with the actuatorunit 21, which has not yet been bonded with the channel unit 4.

In the method for manufacturing bonded substrates according to the firstembodiment as described above, the terminals 51 of the FPC 50 are formedso that the ratio of the height of the core 52 and the thickness of thesolder layer 53 in each terminal 51 is in a range of from 1:1 to 2:1.Accordingly, even when there is a certain degree of variation in theheight of the tip of each core 52 from the lower surface 40 a of the FPC50, the range of the variation in the height of the tip of each core 52and the range of variation in the height of the tip of each terminal 51from the lower surface 40 a of the FPC 50 overlap each other onlyslightly. Thus, when each terminal 51 is bonded with its correspondingland 36, the terminal 51 and the land 36 can be electrically bonded witheach other surely by the effect of absorbing variation in the thicknessdirection of the FPC 50 due to the softening of the solder layer 53 andthe bending of the FPC 50. In addition, there is no fear that height ofthe core 52 is smaller than the thickness of the solder layer 53. Thus,the close adhesion between the core 52 and the solder layer 53 issecured. Further, the heating temperature to bond the FPC 50 with theactuator unit 21 is lower than the melting point of the solder layer 53.Accordingly, the solder layer 53 is not melted into a liquid state butsoftened. Thus, the contact area between the land 36 and the terminal 51increases while the solder layer 53 hardly protrudes out of an areasurrounded by the thermosetting adhesive 54. It is therefore possible toprevent a short-circuit between individual electrodes due to electricconnection between the terminals 51. In addition, since thethermosetting adhesive 54 surrounding the terminals 51, the lands 36 andthe solder layers 53 is cured, the bonding force between each terminal51 and its corresponding land 36 is improved.

When the ratio of the core height to the solder layer thickness is setat 1.5:1 within the range of from 1:1 to 1.8:1, the terminals 51 and thelands 36 abut against each other more surely. This is because thevariation range of the core height and the variation range of theterminal height are prevented from overlapping each other, so that theterminals 51 and the lands 36 can abut against each other surely evenwithout flexibility of the FPC 50. In other words, the FPC 50 hasflexibility in this embodiment so that the terminals 51 and the lands 36are always electrically bonded due to addition of the effect of theflexibility. Further, even if Pb-free solder having a higher meltingpoint than Pb-containing solder is used for the solder layer 53, theterminals 51 and the lands 36 can be electrically connected surely. TheFPC 50 formed in the aforementioned manufacturing method has theaforementioned advantage because the ratio of the height of the core 52to the thickness of the solder layer 53 in each terminal 51 is in arange of from 1:1 to 2:1.

Next, description will be made below on a method for manufacturingbonded substrates according to a second embodiment. FIG. 13 is apartially sectional view of an inkjet head manufactured in the methodfor manufacturing bonded substrates according to the second embodiment.In the same manner as in the first embodiment, the bonded substrates inthis embodiment designates a structure in which an FPC 50 functioning asa first substrate and actuator units 21 functioning as a secondsubstrate are bonded. The manufacturing method of this embodiment is thesame as the method for manufacturing bonded substrates according to thefirst embodiment, except that heating conditions for bonding terminals51′ and lands 36. Parts similar to those in the first embodiment arereferenced correspondingly, and description thereof will be omitted.

When the terminals 51′ of the FPC 50 are bonded with the lands 36 of theactuator unit 21 in this embodiment, the terminals 51′ coated with athermosetting adhesive 54′ are aligned to face the lands 36 in the samemanner as in the first embodiment. The suction portion 91 is moveddownward so as to press each terminal 51′ onto its corresponding land36. At the same time, the FPC 50 and the head body 70 are heated by anot-shown heating apparatus. The pressing is performed in the samemanner as in the first embodiment, but the heating temperature in theheating is different from that in the first embodiment. That is, in thisembodiment, the FPC 50 and the head body 70 are heated to a temperatureequal to or higher than the melting point of a solder layer 53′ andlower than the melting point of the core 52, for example, to 220° C.When the FPC 50 and the head body 70 are heated thus, the solder layer53′ of each terminal 51′ is melted so that integration of the core 52and the land 36 is accelerated as shown in FIG. 13. In this event, thethermosetting adhesive 54′ is present around the solder layer 53′.Accordingly, the molten solder layer 53′ hardly adheres to anotheradjacent terminal 51′. The FPC 50 and the head body 70 are heated at220° C. for a predetermined time. Thus, the thermosetting adhesive 54′progresses to curing.

Next, the FPC 50 and the actuator unit 21 are cooled naturally after apredetermined time has passed. Thus, the terminals 51′ and the lands 36are electrically connected with each other by the solder layer 53′.Further, the FPC 50 and the actuator unit 21 are bonded with each otherby the thermosetting adhesive 54′.

As described above, in the method for manufacturing the bondedsubstrates according to the second embodiment, similar advantages can beobtained in a portion having a similar configuration to that of thefirst embodiment. Further, due to the molten solder layer 53′, thecontact area between each terminal 51′ and its corresponding land 36increases in comparison with that in the first embodiment. When themolten solder layer 53′ is cured, the bonding force between the terminal51′ and the land 36 is enhanced more. In addition, when the terminal 51′and the land 36 are bonded with each other, the solder layer 53′ ismelted so that the effect of absorbing variation due to the softening ofthe solder layer 53′ becomes greater. Accordingly, the terminal 51′ andthe land 36 can be bonded with each other better.

Although the terminals 51, 51′ are coated with the thermosettingadhesive 54, 54′ in each of the aforementioned embodiments, theterminals 51 and the lands 36 may be bonded without applying thethermosetting adhesive thereto, as shown in FIG. 14. In thismodification, the terminals 51 in the first embodiment are not coatedwith the thermosetting adhesive but bonded with the lands 36substantially in the same manner as in the first embodiment. In thiscase, the solder layer 53 is not heated to its melting point. Thus, thesolder layer 53 is hardly melted to flow and stretch to itssurroundings. Since this modification is almost the same as the firstembodiment, almost the same advantages can be obtained. In thismodification, the solder layer 53 is heated to a temperature lower thanits melting point so as not to melt the solder layer 53. However, thesolder layer 53 may be heated to a temperature equal to or higher thanthe melting point of the solder layer 53 and lower than the meltingpoint of the core 52, so that the solder layer 53 can be melted. In thiscase, the thermosetting adhesive 54′ shown in FIG. 13 is not formed.Thus, the contact area between the solder layer 53 and the land 36 isincreased so that the bonding force between the terminal 51 and the land36 is enhanced.

The embodiments of the present invention have been described above.However, the invention is not limited to the aforementioned embodiments.Various changes can be made on the invention without departing from thescope of claims. For example, in each of the aforementioned embodiments,the terminals 51, 51′ of the FPC 50 are coated with the thermosettingadhesive 54, 54′, and bonded with the lands 36 of the actuator unit 21.However, the terminals 51, 51′ of the FPC 50 coated or not coated withthe thermosetting adhesive may be bonded with the lands 36 coated withthe thermosetting adhesive. Although the FPC 50 is moved to approach theactuator unit 21 so as to press the terminals 51, 51′ onto the lands 36,the actuator unit 21 may be moved to approach the FPC 50 so as to pressthe lands 36 onto the terminals 51, 51′. Although each terminal 51includes the solder layer 53, which is 13.3-20 μm thick, this is appliedonly to the case where the terminal is 40 μm high. When the terminalheight is changed desirably, the thickness range of the solder layer 53can be changed suitably. In this case, the thickness of the solder layer53 is set so that the ratio of the changed core height to the changedsolder layer thickness is in a range of from 1:1 to 2:1. Further, thecore 52 may be made of metal other than nickel. The solder layer 53, 53′may be of Pb-containing solder (tin-lead: SnPb) Moreover, the method formanufacturing bonded substrates according to each embodiment isapplicable to various fields other than inkjet heads.

1. A method for manufacturing bonded substrates of a line head printer, in which a plurality of first terminals formed on a first substrate are electrically bonded to a plurality of second terminals formed on a second substrate, the method comprising: forming the first terminals on the first substrate, each of the first terminals having a metal core projecting from a surface of the first substrate, each metal core coated with a solder layer, the solder layer being lower in melting point than the metal core; forming the conductive second terminals on the second substrate; electrically bonding the first terminals to the second terminals by heating the first substrate and the second substrate while applying pressure to the first substrate and the second substrate; and applying a thermosetting adhesive to at least either of the first terminals and the second terminals, wherein: in the forming of the first terminals, a ratio of a height of the metal core from the surface of the first substrate in a thickness direction of the first substrate to a thickness of the solder layer in the thickness direction of the first substrate is in a range of from 1:1 to 2:1, wherein the electrically bonding of the first terminals to the second terminals comprises: heating the first substrate and the second substrate at a temperature, which is equal to or higher than the melting point of the solder layer and is lower than the melting point of the metal core, while applying the pressure to the first substrate and the second substrate, and wherein after the applying of the thermosetting adhesive, the first terminals are electrically bonded to the second terminals by heating the first substrate and the second substrate at a temperature, which is (i) equal to or higher than a curing temperature of the thermosetting adhesive, (ii) equal to or higher than the melting point of the solder layer and (iii) lower than the melting point of the metal core, while applying the pressure to the first substrate and the second substrate.
 2. The method according to claim 1, wherein the first substrate has flexibility.
 3. The method according to claim 1, wherein in the forming of the first terminals, the ratio of the height of the metal core from the surface of the first substrate in the thickness direction of the first substrate to the thickness of the solder layer in the thickness direction of the first substrate is in a range of from 1:1 to 1.8:1.
 4. The method according to claim 1, wherein in the forming of the first terminals, the thickness of the solder layer is in a range of from 13.3 μm to 20 μm.
 5. The method according to claim 1, wherein the solder layer comprises Pb free solder.
 6. The method according to claim 1, wherein the metal core comprises nickel.
 7. The method according to claim 1, wherein the height of the metal core is equal to one half of a result of a width of the metal core minus a width of the opening in the first substrate through which the metal core protrudes.
 8. The method according to claim 1, wherein the thickness of the solder layer is equal to a quotient minus the height of the metal core, wherein the quotient equals a width of the metal core and the solder layer minus a width of the opening in the first substrate through which the metal core protrudes divided by two. 